The present invention relates to a liquid crystal display element and, more particularly, to the configuration of an additional capacitance region which is connected to its pixel electrodes.
FIG. 1 shows a conventional liquid crystal display element of a structure in which liquid crystal 14 is hermetically sealed in the space defined by a pair of opposed transparent substrates 11 and 12 as of glass with a spacer 13 interposed therebetween along their marginal edges. The one transparent substrate 11 has on its inside surface a plurality of pixel electrodes 15 each adjoined by a thin film transistor (hereinafter referred to as TFT) 16 serving as a switching element. The TFT 16 has its drain connected to the pixel electrode 15 corresponding thereto. The other transparent substrate 12 has on its inside surface a transparent common electrode 17 opposite the pixel electrodes 15.
As shown in FIG. 2, the pixel electrodes 15 substantially square in shape are closely arranged in rows and columns on the transparent substrate 11 and gate and source buses 18 and 19 extend adjacent and along the pixel electrodes 15 in the row and column directions, respectively. At each intersection of the gate and source buses 18 and 19 the TFT 16 is disposed, which has its gate connected to the gate bus 18, its source connected to the source bus 18 and its drain connected to the pixel electrode 15.
A voltage is applied across a pair of selected ones of the gate and source buses 18 and 19, the TFT 16 connected thereto at their intersection thus conducts to store charges in the pixel electrode 15 connected to the drain of the conducting TFT 16, and consequently, a voltage is applied across that portion of the liquid crystal 14 lying between the charged pixel electrode 15 and the common electrode 17 to make the liquid crystal 14 permit or inhibit the passage therethrough of light, thus providing a selective display. The display can be erased by discharging the charges stored in the pixel electrode 15.
FIG. 3 is an enlarged plan view showing one pixel and the neighboring portion in a conventional liquid crystal display element, FIG. 4 is a sectional view taken on the line IV--IV in FIG. 3, and FIG. 5 is a sectional view taken on the line V--V in FIG. 3. As shown in FIGS. 3 and 4, the pixel electrode 15 and the source bus 19 of ITO or similar transparent conductive material are formed on the transparent substrate 11, a semiconductor layer 21 as of amorphous silicon is formed which bridges a gap between parallel, opposed marginal portions of the pixel electrode 15 and the source bus 19, and the pixel electrode 15, the source bus 19 and the semiconductor layer 21 are covered with a gate insulating film 22 as of silicon nitride. On the gate insulating film 22 a gate electrode 23 is formed which overlaps the pixel electrode 15 and the source bus 19 through the semiconductor layer 21. The gate electrode 23 is connected at one end to the gate bus 18. Thus, those portions of the pixel electrode 15 and the source bus 19 which are opposite to the gate electrode 23 form a drain electrode 15a and a source electrode 19a, respectively. The electrodes 15a and 19a, the semiconductor layer 21, the gate insulating film 22 and the gate electrode 23 constitute the TFT 16. The gate electrode 23 and the gate bus 18 are simultaneously formed using aluminum, for instance. A protective layer 23 for the liquid crystal is formed on the gate electrode 23 over the entire area of the display screen.
As depicted in FIGS. 3 and 5, one marginal side portion of the pixel electrode 15 extends under the neighboring the gate bus 18 to substantially the center of the bus 18 widthwise thereof to form an additional capacitance region 30 between the extended portion 15b of the pixel electrode 15 and the gate bus 18. The additional capacitance region 30 is needed to supplement the electrostatic capacitance of the pixel electrode 15 to provide a large time constant composed of the electrostatic capacitance of the pixel electrode 15 and the resistance value of a channel region of the TFT 16.
The additional capacitance region 30 is composed of a plurality of divided capacitors. That is, the extended portion 15b of the pixel electrode 15 includes square electrodes 15b1, 15b2 and 15b3, each formed in the shape of an island under the gate bus 18 and connected to the pixel electrode 15 by a bridging segment 32. Electrostatic capacitances formed between the electrodes 15b1, 15b2 and 15b3 and the gate bus 18 are capacitors C.sub.1, C.sub.2 and C.sub.3 depicted in FIG. 2. If in the additional capacitance region 30 a pinhole is made in or dust gets mixed into the gate insulating film 22 between the gate bus 18 and the underlying electrode 15b during manufacture, the insulation between the gate bus 18 and the electrode 15b may sometimes be impaired or shorting may develop therebetween. In such a case, some pixels in the display element always remain in the ON (lighted) state irrespective of an image signal to be displayed, resulting in the quality of the display being impaired. To avoid this, the defective additional capacitance region (i.e. defective one of the divided capacitors) is removed. That is, a focused laser beam is applied through the transparent substrate 12 in FIG. 1 and is brought into a focus 2 to 10 .mu.m in diameter on the bridging segment 32 coupled wish the defective divided capacitor of the additional capacitance region 30 to cut the bridging segment 32 and hence cut the corresponding one of the electrodes 15b1, 15b2 and 15b3 off from the pixel electrode 15.
In the conventional liquid crystal display element, the removal of the defective divided capacitor of the additional capacitance region through laser cutting will reduce the capacitance value of the whole additional capacitance region to 2/3 its set value in the example of FIG. 3. This will cause the potential of the pixel electrode relative to the common electrode to change from its set value, introducing a change in the brightness of the pixel. This does not pose a serious problem in the case of producing a simple black-and-white display, but in the case of providing a high-grade, multi-gradation display, the gradation of the pixel changes, impairing the quality of the display.